The effects of CO co-feed on the catalytic performance of Methanol-to-Hydrocarbons conversion on HZSM-5

Abstract

Methanol-to-hydrocarbons catalysis over HZSM-5 at low methanol partial pressure exhibited low steady-state ethylene and aromatics selectivity at sub-complete methanol conversion. Co-feeding CO increases cumulative carbon ratio of ethylene to the sum of 2-methylbutane and 2-methyl-2-butene (>3×) at the expense of catalyst stability, quantified by >18× increase in site-loss yield, and hydrocarbons selectivity (<80 %). These observations can be rationalized by CO incorporation into the hydrocarbon pool via carbonylation of methyl groups to increase propagation of the aromatics-cycle and of termination sequences relative to that of the olefins cycle. The effects of CO become more prominent at higher partial pressures of CO, at lower inlet concentrations of methanol, olefins, and aromatics, and upon decreasing proton density (per unit volume) and crystallite size in MFI materials. The morphology- and process-dependent effects of CO during MTH on MFI are manifestations of differences in space- and time-averaged relative carbonylation and methylation rates. Both structural catalyst properties and process conditions affect reaction rates during MTH by affecting diffusional constraints, number and contact time of active chain carriers. These results extend to syngas-to-hydrocarbons conversion where high-pressure CO is present to guide the choice of reaction conditions and materials that maximize catalyst lifetime and modulate product selectivity.